Coextrusion of multiple layers of different polymers through a common die is used to produce a variety of polymer products, such as sheet, film, coatings, profiles, pipe, tubing, and foam-core products. Food and beverage packaging is a common application of such coextruded products. Sheet or film, for example, are often shaped after coextrusion into various packaging items by methods such as thermoforming, solid-phase pressure forming, or stamping. Individual polymers exhibit a wide variety of properties, such as permeability to oxygen, water vapor, and other gases and liquids, and chemical resistance to acids, bases, hydrocarbons, oils, alcohols, and other materials. Coextrusion of multiple polymer layers can be used to prepare a product with optimum properties for a particular application.
No comprehensive theory exists for predicting interlayer adhesion in coextrusion of different polymers. Most knowledge comes from trial-and-error testing of the polymers. Different polymers, when coextruded, will typically exhibit good adhesion or no adhesion. Adhesive polymers are commonly used as "tie layers" to bond incompatible polymers that do not normally adhere to each other. Ethylene-vinyl acetate, ethylene-acrylic acid, and ethylene-methyl acrylate copolymers are commonly used as tie layers.
There is currently an increasing emphasis on reducing the volume of plastic packaging waste, and on recycling plastic materials. In the future, it is expected that additional emphasis will be placed on recycling of other plastic articles, such as auto parts. Although coextrusion of multiple polymer layers can be used to prepare a product with optimum properties, such polymer products are also difficult to recycle. Coextrusion of a multi-layer structure that appears to exhibit good adhesion, yet delaminates readily for recycling, would be very useful. It is an object of this invention to provide a coextruded product, of two or more dissimilar polymer layers that appear to exhibit good adhesion, yet will separate readily for recycling.
In particular, it is an object of this invention to provide a coextruded multi-layer structure of a polyketone polymer and an other thermoplastic polymer which appears to adhere to the polyketone polymer, yet separates easily from the polyketone polymer when the integrity of either layer is impaired. Surprisingly, after extensive trial-and-error testing, only five other thermoplastic polymers have been found that meet this objective.